The present invention relates to strain gauge circuits in general, and in particular to a self-calibrating strain gauge circuit with automatic zeroing.
Strain gauges are widely used for the dynamic testing and measuring of objects of all kinds in a variety of applications, including medical applications. One such use of a strain gauge in a medical application is in plethysmography. For example, a plethysmography, a strain gauge is used for measuring and recording changes in the volume of a region of an individual's appendage (finger, arm, leg, foot, etc.) caused by a change in the amount of blood inside the region. As blood flows into the region, the girth of the region increases and as blood flows from the region the girth of the region decreases. This includes the short- term changes caused by the pulse and longer venous changes, often in conjunction with a period of intentional venous constriction. As the changes occur, they are recorded. The magnitude and speed of the changes thus recorded can be used to assess the condition of the circulatory system in the region of the appendage being examined and to diagnose problems that may exist therein.
In the plethysmograph system the strain gauge serves as a transducer. It provides a resistance change which is proportional to any change in the size of the girth of the appendage on which it is fitted. Since, for any given appendage, its length is constant, a change in its volume is proportional to a change in its girth.
To record changes in girth, the strain gauge, in the form of a thin silicon rubber tube filled with an electrically conductive material, is wrapped about the appendage. As blood flows through the appendage, changing the size of the appendage, the girth of the appendage changes. A change in the size of the girth of the appendage stretches or relaxes the tube, depending on whether the blood flows in or from the appendage. The stretching and relaxation of the tube changes the resistance of the material contained in the tube. By measuring the resistance change in the tube, an output proportional to the flow of blood in the appendage is obtained.
In a relaxed condition a strain gauge has a predetermined resistance. When the gauge is placed on an appendage, such as an arm or leg, it is stretched. The amount of the stretching of the gauge may differ each time it is used, but nevertheless the stretching results in a change in the gauge resistance. The magnitude of the change in the gauge resistance depends on the size of the appendage and the amount of the stretching when the gauge is fitted. A change in resistance also occurs when the gauge is repeatedly placed on a particular appendage as well as when it is moved from one appendage to another. Because of these initial gauge resistance changes when a gauge is fitted, in order for an operator using a particular gauge to compare readings obtained during successive uses of the gauge or to compare readings obtained from different gauges, it is necessary to calibrate gauges each time they are used.
Another feature of known conventional strain gauge circuits involves variations in the initial or nominal size of a strain gauge when it is fitted to objects of widely differing sizes. This affects the scale required of an output indicator. In the absence of suitable calibration of the output indicator, the gauge input may readily exceed the range of the indicator. For example, as a given gauge is fitted to appendages of successively larger girth, the initial or nominal resistance of the gauge is higher each time. If the scale of the indicator is insufficient to handle possible excursions relative to each nominal resistance, accurate readings are not possible. It, therefore, is highly desirable to have a circuit which normalizes the output of a gauge independently of its resistance.
Heretofore the calibration of strain gauges in general, and those used in strain gauge plethysmography in particular, have been tedious and time-consuming. Conventional calibration was not done on an object to be examined but typically involved the use of models. And, in general, it has required burdensome record keeping to record the necessary figures.